1. Field of the Invention
The present invention relates to an X-ray image intensifier and, more particularly, to an X-ray image intensifier suitable for low-energy X-ray photography.
2. Description of the Related Art
An X-ray image intensifier is used to obtain an X-ray transmitted image of an object to be examined in an X-ray diagnostic apparatus or the like. The material constituting an input window of the X-ray image intensifier is naturally required to transmit x-rays well and cause less scattering of x-rays. In addition, since the interior of the X-ray image intensifier is in a vacuum, the input window is required to have an enough strength to withstand this vacuum. The input window is also required to be well balanced in cost with other components that constitute the X-ray image intensifier.
Aluminum or titanium has been used as the material of the input window, that can meet the above conditions.
The X-ray image intensifiers are presently, widely used in the fields of, e.g., medical diagnoses and industrial applications (e.g., nondestructive testing). These X-ray image intensifiers have various excellent features that they can convert a moving x-ray image into a visible image in real time and can reduce the exposure dose compared to systems using films. This extends the range of applications of the X-ray image intensifiers.
Recently, in the field of medical diagnoses, researchers have investigated the use of the X-ray image intensifier in diagnoses of breast cancers. Since the diagnosis of a breast cancer is testing on a soft tissue, relatively low-energy X-rays, such as those generated at an X-ray intensifier voltage of approximately 20 KV to 40 KV, are used. Also, in the field of industrial applications (e.g., nondestructive testing), researchers have investigated the use of low-energy X-rays in testing on products made of paper or resins.
In applying low-energy X-rays, the transmittance of an input window of the X-ray image intensifier with respect to X-rays is of primary concern. The X-ray transmittance is determined by the material and the thickness of an input window and the energy of X-rays: the higher the energy of X-rays or the smaller the thickness of a window, the higher the transmittance. Also, an element having a smaller atomic number generally has a higher x-ray transmittance.
FIG. 1 shows the X-ray transmittances of aluminum and titanium used as the material of the input window of the X-ray image intensifier. Referring to FIG. 1, the X-ray transmittance (%) is plotted on the ordinate, and the X-ray energy (KeV) is plotted on the abscissa. A broken curve a indicates the X-ray transmittance of aluminum (thickness 1 mm), and a solid curve b indicates that of titanium (thickness 250 .mu.m).
When titanium is to be used as the material of the input window, the thickness of titanium must be decreased to about 250 .mu.m in order to obtain a practically sufficient X-ray transmittance. An input window consisting of a material with such a small thickness in a finished X-ray image intensifier assumes a recessed outer appearance owing to a differential pressure with respect to the atmospheric pressure, because the interior of the tube is in a vacuum. To assemble this titanium input window into the X-ray image intensifier, the input window is joined with a stainless-steel ring by spot welding. This stainless-steel ring is then welded to an Fe--Ni--Co alloy ring which is in turn joined with a glass container as a part of an envelope.
When aluminum is used as the material of the input window, on the other hand, the input window is welded to a stainless-steel ring at a predetermined temperature and under a predetermined pressure. Since, however, the mechanical strength of aluminum is smaller than that of titanium, the thickness of aluminum is set to about 1 mm. In addition, in order to withstand the atmospheric pressure, the outer surface of the input window is projected.
As shown in FIG. 1, the transmittance of aluminum is higher than that of titanium for x-rays with a low energy of 20 KeV to 40 KeV. Therefore, when the energy of X-rays is low, aluminum is a more suitable material of the input window than titanium. Aluminum materials are classified into various types in accordance with the types and the amounts of additive substances contained in them and the conditions of treatments. These materials are also different in mechanical and thermal characteristics. According to literature such as "Aluminum HandBook," the aluminum materials are classified as follows: (A.A. shows a grade determined by the Aluminum Association, Inc.)
A.A.#1000 type: Pure aluminum. Aluminum with a purity of 99% or more. Processability, corrosion resistance, and weldability are high, but strength is low. PA0 A.A.#2000 type: Al--Cu alloy. Duralumin. Strength is high, but corrosion resistance is poor. PA0 A.A.#3000 type: Al--Mn alloy. Strength is slightly increased by adding Mn to the #1000 type. PA0 A.A.#4000 type: Al--Si alloy. Abrasion resistance and heat resistance are improved by addition of Si. PA0 A.A.#5000 type: Al--Mg alloy. Strength is high. PA0 A.A.#6000 type: Al--Mg--Si alloy. Both strength and corrosion resistance are high. PA0 A.A.#7000 type: Al--Zn--Mg alloy. Strength is highest of all aluminum alloys, but formability is poor.
Conditions required for the material constituting an input window of an X-ray image intensifier are as follows:
(a) Having an enough strength to withstand the atmospheric pressure.
(b) Having an enough strength to withstand the atmospheric pressure not only at room temperature but at the baking temperature (200.degree. C. to 400.degree. C.) in an exhaust step as one of the steps of manufacturing X-ray image intensifiers.
(c) Having a sufficient corrosion resistance.
(d) Having a high formability in order to form the input window into a projecting shape.
Aluminum materials meeting these conditions are those of the A.A.#5000 type and the A.A.#6000 type, and these materials are actually used.
As described above, the input window is joined to the stainless-steel ring in the process of manufacturing the X-ray image intensifier. In this case, the aluminum material constituting the input window and the stainless-steel ring are joined together at a temperature of 400.degree. C. or more and under a predetermined pressure. This joining is performed by diffusing molecules of the aluminum material and the stainless steel into each other at a high temperature and a high pressure. Since the temperature of the aluminum material rises, a certain change occurs inside the aluminum material. As an example, in the case of the A.A.#6000 type aluminum material, an Mg.sub.2 Si precipitate forms while the temperature falls from the high temperature in the joining, and in the step of baking at about 250.degree. C. This state will be described with reference to FIGS. 2A and 2B.
Generally, when aluminum is kept at a high temperature, Al crystal grains grow into coarse grains 21 as shown in FIG. 2A, and, in the middle of cooling from the high temperature, Mg.sub.2 Si phases 22 precipitate in grain boundaries as shown in FIG. 2B (especially in the A.A.#6000 type). This precipitate 22 is different in X-ray transmittance from aluminum. This difference in X-ray transmittance between the precipitate and aluminum is negligibly small when the energy of X-rays is 50 KV or more, if, however, the X-ray energy becomes 30 KV or less, the difference in X-ray transmittance between the two increases. Consequently, even when uniform X-rays are incident on the input window, the amount of transmitted X-rays changes in accordance with the presence/absence of the precipitate.
When, therefore, an aluminum material containing the precipitates is directly formed into an input window, the presence of the precipitates gives rise to unevenness corresponding to the distribution of the precipitates in a visible-light image produced by the X-ray image intensifier. In addition, in the A.A.#5000 type aluminum material, recrystallization of aluminum occurs at the high temperature to produce coarse crystal grains of aluminum inside the aluminum material. The coarse crystal grain of aluminum is different in crystal orientation from the surrounding aluminum. Therefore, X-ray diffraction conditions vary in accordance with the incident direction of X-rays, and this produces a difference in X-ray transmittance between the two types of aluminum. Also in this case, when the energy of X-ray becomes low, a large difference arises in X-ray transmittance between a portion containing the coarse crystal grains and a portion not containing them, as in the A.A.#6000 type aluminum.
Assuming that the thickness of aluminum is t.sub.1, an X-ray transmittance T.sub.1 is represented by: EQU T.sub.1 =exp(-.mu.t.sub.1)
Likewise, assuming that the thickness of aluminum is t.sub.2, an X-ray transmittance T.sub.2 is given by: EQU T.sub.2 =exp(-.mu.t.sub.2)
The ratio of one transmittance to the other is, therefore, T.sub.1 /T.sub.2 =exp[.mu.(t.sub.2 -t.sub.1)].
In the above relation, .mu. is the transmittance coefficient corresponding to the energy of X-rays.
Note that if t.sub.2 &gt;t1, the value of .mu.(t.sub.2 -t.sub.1) is positive, and so T.sub.1 /T.sub.2 &gt;1. In addition, since .mu. increases as the X-ray energy decreases, T.sub.1 /T.sub.2 increases when the X-ray energy decreases. That is, the lower the energy of X-rays, the larger the transmittance difference (ratio). It is assumed that this relationship produces unevenness in a visible-light image produced by the X-ray image intensifier.
Recently, image processing apparatuses are widely used, and even a slight difference in X-ray transmittance is emphasized in these apparatuses. Hence, there is a high possibility that unevenness in a produced image, which is supposed to result from precipitates or coarse crystal grains as described above, becomes a more serious obstacle in the future.